feat(admin): add Platform Administration section to sidebar

Reorganized sidebar navigation:
- Added "Platform Administration" section header below Dashboard
- Grouped Companies, Vendors, Users, Customers, Marketplace under it
- Added new Customers page (empty placeholder for future)
- Changed Marketplace icon from shopping-bag to globe
- Renamed "Marketplace Import" to "Marketplace" for brevity

New files:
- app/templates/admin/customers.html
- Route: GET /admin/customers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/templates/admin/customers.html

@@ -0,0 +1,51 @@
+{# app/templates/admin/customers.html #}
+{% extends "admin/base.html" %}
+
+{% block title %}Customers{% endblock %}
+
+{% block alpine_data %}adminCustomers(){% endblock %}
+
+{% block content %}
+<!-- Page Header -->
+<div class="flex items-center justify-between my-6">
+    <div>
+        <h2 class="text-2xl font-semibold text-gray-700 dark:text-gray-200">
+            Customers
+        </h2>
+        <p class="text-sm text-gray-600 dark:text-gray-400 mt-1">
+            Manage platform customers
+        </p>
+    </div>
+</div>
+
+<!-- Empty State -->
+<div class="px-4 py-12 bg-white rounded-lg shadow-md dark:bg-gray-800">
+    <div class="text-center">
+        <span x-html="$icon('user-group', 'mx-auto h-12 w-12 text-gray-400')"></span>
+        <h3 class="mt-4 text-lg font-medium text-gray-900 dark:text-gray-100">
+            Customers Management
+        </h3>
+        <p class="mt-2 text-sm text-gray-500 dark:text-gray-400">
+            Customer management features coming soon.
+        </p>
+        <p class="mt-1 text-sm text-gray-500 dark:text-gray-400">
+            This section will allow you to view and manage customers across all vendors.
+        </p>
+    </div>
+</div>
+{% endblock %}
+
+{% block extra_scripts %}
+<script>
+function adminCustomers() {
+    return {
+        ...data(),
+        currentPage: 'customers',
+
+        init() {
+            console.log('Customers page initialized');
+        }
+    };
+}
+</script>
+{% endblock %}